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High Temperature Applicable Separator by Using Polyimide Aerogel/Polyethylene Double-Layer Composite Membrane for High-Safety Lithium Ion Battery

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High Temperature Applicable Separator by Using Polyimide Aerogel/Polyethylene Double-Layer Composite Membrane for High-Safety Lithium Ion Battery Int. J. Electrochem. Sci., 14 (2019) 7133 – 7148, doi: 10.20964/2019.08.18 International Journal of ELECTROCHEMICAL SCIENCE www.electrochemsci.org High Temperature Applicable Separator by Using Polyimide aerogel/polyethylene Double-Layer Composite Membrane for High-Safety Lithium Ion Battery Gunhwi Kim#, Jinyoung Kim#, Jongyeob Jeong, Daero Lee, Myeongsoo Kim, Sangrae Lee, Seohyun Kim , Hyunju Lee, Haksoo Han* Department of Chemical and Biomolecular Engineering, Yonsei University, 262 Seongsan-no, Seodaemun-gu, Seoul 120-749, South Korea E-mail: [email protected] #These two authors have equally contributed to this study Received: 5 March 2018 / Accepted: 16 July 2018 / Published: 30 June 2019 To address the vulnerability of a conventional separator to high temperature, a new double-layer function of separator with a double-layer function is suggested for high-safety lithium-ion batteries that are used in harsh condition. The new composite separator was fabricated by coating thermally imidized polyimide aerogel (PIA) on a porous polyethylene (PE) membrane, with polyvinylidene fluoride (PVDF) as a binder, to improve the thermal stability of the separator. The PIA/PE double-layer composite performed very well, especially in terms of thermal stability. The newly suggested separator showed a near-zero thermal shrinkage rate compared to commercial PE separators, which are defective in having high levels of the same, and retained its structure up to 140 °C. The PIA supporting layer showed hardly any change after heat treatment, and the PE layer performed its role as a shut-down layer perfectly, ensuring the safety of the new separator. These results prove that a PIA/PE separator can prevent batteries from exploding and overcharging. In addition, the PIA/PE separator also demonstrated excellent electrolyte uptake and electrolyte wettability to electrolyte. PIA/PE separator based coin cells exhibited outstanding cycling and rate performances, especially at high current rates, compared to coin cells with a standard PE separator. Therefore, the new PIA/PE separator is an ideal candidate for use in high-safety lithium-ion batteries at high temperatures, based on its excellent thermal and chemical stability. Keywords: Polyimide aerogel; Separator; Composite; Lithium-ion battery; Safety; Thermal stability. Int. J. Electrochem. Sci., Vol. 14, 2019 7134 1. INTRODUCTION Lithium ion batteries have recently been highly sought after and are used in various industries [1]. To keep up with the rapid developments in technology, high-performance lithium-ion batteries with higher capacities are in strong demands [1–4]. To achieve this, researchers demonstrated various approaches to thin the separator and invest more in the cathode and anode [5–7]. However, they neglected to improve safety thresholds to accommodate their efforts to amplify performance. The result was many accidents related to explosions of lithium batteries; these were important issues in 2016, and revealed the vital role of the separator in battery systems [8–10]. A separator works in a battery system as an electron pathway and as a partition between the cathode and anode, thus preventing failures due to short-circuits [11,12]; it is also considered the main suspected cause for explosions of the secondary cell. Polyethylene and polypropylene porous membranes are the polyolefin materials mostly commonly used as commercial separators [13,14]. Although porous membranes are good candidates for separators with great chemical stability and mechanical properties, their melting point ranges from 130 °C to 160 °C, which is too low to be applied in a high temperature environment. They also have several drawbacks, such as a low thermal stability, which sometimes leads to shrinkage occurring below the melting point, and a low liquid electrolyte wettability. Therefore, in special situations such as operation in hot environments or overheating caused by overcharging, their low thermal stability induces dimensional shrinkage [15–17]. In addition, the contraction of the separator can also cause contact between the cathode and anode, which leads to a dangerous explosion or a conflagration of the secondary cell system. Therefore, blocking its own pores to prevent such contact, and thus shutting down the operating system in such a situation, is a significant requirement of a separator [18–20]. Separators do not seriously affect electrical performance; however, they have a significant role in terms of safety. Firstly, a separator should be able to block its pores at high temperatures, without contracting, to halt electron transfer. In addition, high porosity, electrolyte wettability, and excellent electrolyte uptake are required in an efficient separator [21–24]. To satisfy these requirements, a polyimide aerogel/polyethylene (PIA/PE) separator was fabricated by coating PIA with a high porosity and good thermal stability on a PE membrane layer with polyvinylidene fluoride (PVDF) as a binder. When overcharged, the temperature reaches the melting point of PE (about 130 °C) and the membrane starts to shrink. PE melts completely, becomes colorless, and remains as a collapsed structure. In contrast, the PE layer of a PIA/PE separator melts and blocks its pores, while the PIA’s high thermal stability of PIA prevents the shrinkage of the PIA/PE membrane. Researchers have continuously tried to apply Polyimide (PI) to fabricate a secondary cell separator with a high thermal and chemical stability[25–27]. However, existing research on applying polyimide to the fabrication of separators produced separators mainly by dissolving chemically imidized PI powder and remolding it, to bypass the problem of the low thermal stability of the shutdown layer, which is incompatible with the thermal curing process of traditional PI. However, the fabricated PI layer in this study has a relatively lower porosity and can be re-dissolved easily [28–30]. Therefore, to improve these properties, the PIA used here was fabricated using thermal curing, mixed with the PVDF binder and was coated on the PE layer. These steps stabilized the double-layer Int. J. Electrochem. Sci., Vol. 14, 2019 7135 membrane. Repeated measurements, indicated that the newly developed separator showed excellent behavior in terms of thermal, chemical, and physical properties. 2. EXPERIMENTAL 2.1 Materials Forthis study, pyromellitic dianhydride (PMDA) and 4,4′-oxydianiline (ODA), with purity > 98%, were bought from Tokyo Chemical Industry Co. Ltd (Tokyo, Japan). Anhydrous 1-methyl-2- pyrrolidinone (NMP) and with purity > 99% was purchased from Duksan Pure Chemicals Co. Ltd (Gyeonggi-Do, Korea). A bolt closure-type autoclave (Ilshin, Daejeon, Korea), capable of carrying out experiments up to 11.7MPa and 250°C, was used. PVDF was purchased from Sigma Aldrich (Darmstadt, Germany). LiPF6 1M EC/DEC (1:1 v/v), which was used as the electrolyte, was purchased from Soulbrain Co.Ltd (Seongnam, Korea). Polyethylene membrane (t16-518) was purchased from SK (Gyeonggi-Do, Korea). 2.2 Fabrication of polyimide aerogel Poly (amic acid) solution, the precursor of polyimide, was prepared by reacting PMDA (2.18 g, 10 mmol) and ODA (2.0024 g, 10 mmol) with NMP (20 mL) as the solvent. ODA was first dissolved in NMP, and PMDA was then added to the solution to prepare the PAA solution. The PAA solution was synthesized by polymerizing of PMDA and ODA. It was then stirred for 24 h at room temperature (20 °C) under a nitrogen (N2) atmosphere. This solution was the precursor of the polyimide aerogel. The PAA solution was placed in the autoclave in 30 mL glass vial. The space between the glass vial and the autoclave was filled with acetone halfway up the height of the bottle, to create a high- pressure atmosphere in the autoclave during the curing process. The autoclave system was sealed airtight and put into a curing oven. The temperature profile of the curing process is set as follows: 180 °C at a ramp rate of 2 °C/min, maintained at 180 °C for 6 h, and cooled at a ramp rate of 2 °C/min. After the curing process, the glass bottle was put into a vacuum oven and dried under a vacuum at 80 °C for 24 h to completely remove residual solvent. 2.3 Preparation of composite separators The composite separators were fabricated using a solution casting method. The PE separator was used as a coating substrate. Aerogel particles used as a supporting layer and the PVDF binder were prepared at a weight ratio of 9:1, and mixed with NMP. The resulting slurry was stirred in a clay pot for 30 min. The slurry was then coated onto one side of the porous PE separator using a doctor blade to ensure uniform thickness. We coated only one side of the PE membrane because it was enough to prevent the composite membrane from shrinking at a high temperature. The final product was dried under vacuum at 80 °C for 24 h to obtain the PIA/PE separator. Int. J. Electrochem. Sci., Vol. 14, 2019 7136 2.4 Electrode preparation and cell assembly Lithium metal was used as the anode. The cathode was composed of 90 wt% LiCoO2, 6 wt% Super-P, and 4 wt% PVDF. The NMP-based cathode slurry was coated on an aluminum foil and dried overnight in a vacuum oven at 80 °C. 2032-type coin cells were assembled by placing the anode– separator–cathode sequence in an Argon conditioned glovebox. An electrolyte solution was prepared containing 1 M LiPF6 and a solvent mixture of ethylene carbonate (EC), dimethyl carbonate (DMC), (EC/DMC = 1/1 wt/wt, PuriEL, Soulbrain Co., Ltd., Korea). 2.5 Characterization of the separators Field-emission scanning electron microscopy (FE-SEM, JEOL-7001F) was used to investigate the surface and cross-section’s morphological characteristics. Images of the separator treated at a high temperature were also observed to verify its shut-down function. The membrane was frozen in liquid nitrogen and broken or cut to allow observations of the smooth cross-section images of the separators.
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